1. Field of the Invention
The present invention relates to a glass-ceramics for magnetic disc substrates, a magnetic disc substrate and a magnetic disc.
2. Related Art Statement
Recently, a magnetic disc substrate made of glass is practically used in a part of the field of technology. An especially high strength is requested for a magnetic disc substrate for HDD use. Meanwhile, a glass has usually so low strength that it has not a sufficient reliability as a magnetic disc substrate for HDD use. Thus, a tempered glass, such as a chemically tempered glass or a glass ceramics, etc. has to be used. However, if a chemically tempered glass, such as a soda-lime glass, etc. is used as a material for a magnetic disc substrate, alkali metal ions contained in the substrate are likely dissolved out to corrode the magnetic film of the magnetic disc.
Therefore, the inventor has made studies on using a glass-ceramics instead of the chemically tempered glass, as the material for magnetic disc substrate. In a glass-ceramics, almost all the alkali metal ions contained in the glass are existent in the crystal phase of the glass, and only minor amounts of the alkali metal ions are existent in the glass matrix, hence there arises no problem that the alkali metal components constituting the glass ceramics are dissolved out to corrode the magnetic film of the magnetic disc.
Meanwhile, because the chemically tempered glass is wholly composed of an amorphous phase, the surface of the glass can be polished to a center line average surface roughness (Ra) of around 6 .ANG., if the surface of the glass is subjected to a polishing treatment in order to use the glass as the magnetic disc substrate. To the contrary, the glass-ceramics has a crystal phase and an amorphous phase which have different hardness from each other. Thus, even after subjected to the polishing treatment, the glass-ceramics has unavoidably minute undulations between the crystal phase and the amorphous phase. As a result, it was difficult to suppress the center line average surface roughness (Ra) of the polished surface to 20 .ANG. or less.
In order to solve the disadvantageous problem, the inventor made many studies leading to the present invention to find out that the center line average surface roughness (Ra) of the polished surface after the precise polishing treatment can be reduced to a level of 20 .ANG. or less, if a specific Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 series glass-ceramics is used, and concretely disclosed a magnetic disc substrate using the glass-ceramics in the specification of Japanese Patent Application No. 7-174,895.
Even after the disclosure of the finding, there is an increasing demand for recording a large volume of informations, particularly image informations, in a more compact magnetic disc, accompanying with the development of multi-media communication, and a more improved recording density of the magnetic disc has been earnestly desired. As a result, particularly in the reading/writing zone of the magnetic disc, a decreased center line average surface roughness (Ra) of an extent of 10 .ANG. or less has been eagerly desired. Such a desire is extremely difficult to achieve in the magnetic disc substrate consisting of the Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 series glass-ceramics. Though the center line average surface roughness (Ra) of an extent of 10 .ANG. or less of the polished surface of the magnetic disc substrate after the precise polishing treatment may be achieved for a non-glass-ceramics or a glass-ceramics of a low crystallization percentage, the strengths of such glasses are relatively low to incur several disadvantages.
For a glass for use as a magnetic disc substrate, there is also a material having a coefficient of thermal expansion of (80-100).times.10.sup.-7 /k, preferably (85-90).times.10.sup.-7 /k, at 100.degree.-300.degree. C., which is now used as a practical standard material. This is because the drive shaft that supports the magnetic disc substrate is made of SUS of a coefficient of thermal expansion of (80-100).times.10.sup.-7 /k at 100.degree.-300.degree. C. and may cause deformation of the substrate to incur troubles at the time of recording and/or reproducing, if the coefficient of thermal expansion of the substrate greatly differs from that of the drive shaft. Thus, in designing a composition of such a glass, an attention should essentially be paid that the glass may have the coefficient of thermal expansion within the above range.
Also, in the Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 series glass-ceramics for magnetic disc substrate use, there is a problem in that a glass can hardly be obtained having a significantly decreased center line average surface roughness (Ra) after a precise polishing treatment of the surface and which is capable of controlling the coefficient of thermal expansion within the range of (80-100).times.10.sup.-7 /k at 100.degree.-300.degree. C., while maintaining a desired strength.
The inventor has made many studies and experiments on the Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 series glass-ceramics but could hardly solve the above problem for several years. For instance, Japanese Patent Application Laid-open No. 329,440/94 discloses a method of controlling the surface roughness of the Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 series glass-ceramics, but the method could not decrease the center line average surface roughness (Ra) to a level of 10 .ANG. or less after a precise polishing treatment of the surface.
Also, the inventor disclosed in Japanese Patent Application No. 174,895/95 (now, published as Japanese Patent Application Laid-open No. 35,234/97) a Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 series glass-ceramics having a specific composition. However, even in this glass-ceramics, a glass can hardly be obtained having a significantly decreased center line average surface roughness (Ra) of a level of 10 .ANG. or less, particularly 8 .ANG. or less, after a precise polishing treatment of the surface, while simultaneously controlling the coefficient of thermal expansion within the range of (80-100).times.10.sup.-7 /k at 100.degree.-300.degree. C.
Therefore, it is an object of the present invention to provide a Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 series glass-ceramics for magnetic disc substrate use having a significantly decreased center line average surface roughness (Ra) after a precise polishing treatment of the surface and which is capable of controlling the coefficient of thermal expansion within the range of (80-100).times.10.sup.-7 /k at 100.degree.-300.degree. C., while maintaining a desired strength.